Modified polytetrafluoroethylene, molded product, and method for producing stretched porous material

ABSTRACT

wherein R1 represents a hydrogen atom or an alkyl group, L represents a single bond, —CO—O—*, —O—CO—*, or —O—, * represents a bonding position to R2, and R2 represents a hydrogen atom, an alkyl group, or a nitrile group.

TECHNICAL FIELD

The present invention relates to a modified polytetrafluoroethylene, amolded product and a method for producing a stretched porous material.

BACKGROUND ART

Polytetrafluoroethylenes are used for various applications because oftheir excellent properties.

Among them, with respect to a modified polytetrafluoroethylene usingtetrafluoroethylene and another monomer, various studies have been made(Patent Document 1).

PRIOR ART DOCUMENT Patent Document

Patent Document 1: WO2009/137736

DISCLOSURE OF INVENTION Technical Problem

In recent years, a modified polytetrafluoroethylene excellent inbreaking strength is demanded.

Here, usually at the time of producing a modifiedpolytetrafluoroethylene, a fluorinated surfactant is used. However, inrecent years, from the viewpoint of environmental problems, use of afluorinated surfactant has been restricted. Therefore, a modifiedpolytetrafluoroethylene which can be produced without using afluorinated surfactant, is desired.

The present invention has an object to provide a modifiedpolytetrafluoroethylene excellent in breaking strength.

Further, the present invention has an object to provide a molded productand a method for producing a stretched porous material.

Solution to Problem

The present inventors have made intensive studies to achieve the aboveobjects, and as a result, have found it possible to achieve the aboveobjects by the following constructions.

(1) A modified polytetrafluoroethylene comprising a polymer having unitsbased on tetrafluoroethylene, and a polymer having units based on amonomer represented by the formula (1), wherein

the content of the units based on a monomer represented by the formula(1) is from 10 to 500 mass ppm, to all units in the modifiedpolytetrafluoroethylene, and

the standard specific gravity is from 2.155 to 2.175,

CH₂═CR¹-L-R²  Formula (1)

wherein R¹ represents a hydrogen atom or an alkyl group, L represents asingle bond, —CO—O—*, —O—CO—* or —O—, * represents a bonding position toR², and R² represents a hydrogen atom, an alkyl group or a nitrilegroup.(2) The modified polytetrafluoroethylene according to (1), wherein thecontent of the units based on a monomer represented by the formula (1)is from 30 to 100 mass ppm, to all units in the polytetrafluoroethylene.(3) The modified polytetrafluoroethylene according to (1) or (2),wherein the monomer represented by the formula (1) is a monomer selectedfrom the group consisting of a monomer represented by the formula (1-1),a monomer represented by the formula (1-2), a monomer represented by theformula (1-3), and a monomer represented by the formula (1-4),

CH₂═CR¹—CO—O—R³  Formula (1-1)

CH₂═CR¹—O—CO—R⁴  Formula (1-2)

CH₂═CR¹—O—R⁵  Formula (1-3)

CH₂═CR¹—R⁶  Formula (1-4)

wherein R¹ represents a hydrogen atom or an alkyl group, R³ represents ahydrogen atom or an alkyl group, R⁴ represents an alkyl group, R⁵represents an alkyl group, and R⁶ represents a nitrile group.(4) The modified polytetrafluoroethylene according to any one of (1) to(3), wherein the monomer represented by the formula (1) is a monomerrepresented by the formula (1-1).(5) The modified polytetrafluoroethylene according to any one of (1) to(4), wherein the modified polytetrafluoroethylene is in a particulateform, and its average primary particle size is from 0.20 to 0.30 μm.(6) The modified polytetrafluoroethylene according to any one of (1) to(5), which is for paste extrusion molding.(7) A molded product formed by paste extrusion molding of the modifiedpolytetrafluoroethylene as defined in any one of (1) to (6).(8) A method for producing a stretched porous material, which comprisessubjecting the modified polytetrafluoroethylene as defined in any one of(1) to (6) to paste extrusion to obtain an extruded bead, and stretchingthe extruded bead to obtain a stretched porous material.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a modifiedpolytetrafluoroethylene excellent in breaking strength.

Further, according to the present invention, it is possible to provide amolded product using the modified polytetrafluoroethylene, and a methodfor producing a stretched porous material.

DESCRIPTION OF EMBODIMENTS

Meanings of Terms in the Present Invention are as Follows.

A “unit” is a general term for an atomic group derived from one moleculeof a monomer, formed directly by polymerization of the monomer. To allunits which a polymer comprises, the content of each unit (mass %) isobtainable by analyzing the polymer by a solid-state nuclear magneticresonance spectra (NMR) method, but it can be estimated from the chargedamount of each monomer. Usually, the content of each unit calculatedfrom the charged amount of each monomer is substantially coincident withthe actual content of each unit.

A characteristic point of a modified polytetrafluoroethylene of thepresent invention (hereinafter referred to also as a “modified PTFE”) issuch a point that it comprises a polymer having units (hereinafterreferred to also as “TFE units”) based on tetrafluoroethylene(hereinafter referred to also as “TFE”) and a polymer having units(hereinafter referred to also as “units A”) based on a monomer(hereinafter referred to also as a “monomer A”) represented by thelater-described formula (1) to be contained in a predetermined amount.Surprisingly, the modified PTFE of the above composition is excellent inbreaking strength.

As will be described later, the modified PTFE of the present inventioncan be produced without using a fluorinated surfactant.

Hereinafter, a polymer having units A will be referred to also as a“polymer A”.

<Modified PTFE>

The modified PTFE of the present invention comprises a polymer havingTFE units, and a polymer having units A.

The modified PTFE of the present invention may be a mixture of a polymerhaving TFE units and a polymer having units A, or it may be a mixturefurther containing a copolymer comprising TFE units and units A. As thepreferred production method for modified PTFE as described belowsuggests, it is considered that the modified PTFE of the presentinvention is not only composed of only a mixture of the respectivelyindependent polymers i.e. the polymer having TFE units and the polymerhaving units A, but may sometimes contain a copolymer having TFE unitsand units A.

(Polymer Having TFE Units)

The modified PTFE contains a polymer having TFE units.

The modified PTFE usually contains a polymer having TFE units, as themain component. The main component is meant that to all units in themodified PTFE, the content of the polymer having TFE units is at least99.700 mass %, preferably at least 99.900 mass %.

(Monomer Represented by the Formula (1))

The modified PTFE contains a polymer having units A derived from amonomer A, i.e. a polymer A.

CH₂═CR¹-L-R²  Formula (1)

R¹ represents a hydrogen atom or an alkyl group. The number of carbonatoms in the alkyl group is preferably from 1 to 3, more preferably 1.

L represents a single bond, —CO—O—*, —O—CO—* or —O—. * represents abonding position to R². For example, when L is a —CO—O—*, the formula(1) represents CH₂═CR¹—CO—O—R².

R² represents a hydrogen atom, an alkyl group or a nitrile group.

The number of carbon atoms in the alkyl group is preferably from 1 to10, more preferably from 1 to 6, further preferably from 1 to 4.

The alkyl group may be linear or may be cyclic. When the alkyl group iscyclic, it corresponds to a cycloalkyl group.

The monomer A is preferably a monomer selected from the group consistingof a monomer represented by the formula (1-1), a monomer represented bythe formula (1-2), a monomer represented by the formula (1-3), and amonomer represented by the formula (1-4).

CH₂═CR¹—CO—O—R³  Formula (1-1)

CH₂═CR¹—O—CO—R⁴  Formula (1-2)

CH₂═CR¹—O—R⁵  Formula (1-3)

CH₂═CR¹—R⁶  Formula (1-4)

The definition of R¹ is as described above.

R³ represents a hydrogen atom or an alkyl group, preferably an alkylgroup having from 1 to 6 carbon atoms.

R⁴ represents an alkyl group, preferably an alkyl group having from 1 to3 carbon atoms, more preferably a methyl group.

R⁵ represents an alkyl group, and is preferably a linear alkyl group ora cyclic alkyl group.

R⁶ represents a nitrile group.

The monomer A may, for example, be methyl acrylate, methyl methacrylate,ethyl acrylate, ethyl methacrylate, propyl acrylate, propylmethacrylate, butyl acrylate, butyl methacrylate, hexyl methacrylate,cyclohexyl methacrylate, vinyl methacrylate, vinyl acetate, acrylicacid, methacrylic acid, acrylonitrile, methacrylonitrile, ethyl vinylether, or cyclohexyl vinyl ether.

As the fluorine-free monomer, a monomer represented by the formula (1-1)and a monomer represented by the formula (1-2) are preferred, and amonomer represented by the formula (1-1) wherein R³ is an alkyl grouphaving from 1 to 6 carbon atoms is particularly preferred.

The content proportion of units A in the modified PTFE is from 10 to 500mass ppm, to the content of units based on all monomers in the modifiedPTFE. If the content of units A is less than 10 mass ppm, or exceeds 500mass ppm, the breaking strength of the modified PTFE will be inferior.

Especially, from such a viewpoint that the breaking strength of themodified PTFE will be more excellent, the content of units A ispreferably from 10 to 200 mass ppm, more preferably from 20 to 200 massppm, further preferably from 30 to 150 mass ppm, particularly preferablyfrom 30 to 100 mass ppm.

As the monomer A, one type may be used alone, or two or more types maybe used in combination. In the case of using two or more monomers A, thetotal content of units A based on the respective monomers A may bewithin the above range.

The modified PTFE may contain units other than TFE units and units A, ina range not to impair the effects of the present invention.

Here, the total content of TFE units and units A is preferably at least99.700 mass %, more preferably at least 99.800 mass %, to all units inthe modified PTFE. As the upper limit, 100 mass % may be mentioned.

The standard specific gravity (hereinafter referred to also as “SSG”) ofthe modified PTFE is from 2.155 to 2.175. Especially, from such aviewpoint that the breaking strength of the modified PTFE will be moreexcellent, it is preferably from 2.155 to 2.170, more preferably from2.160 to 2.170.

SSG is an index for the molecular weight, and the larger the SSG, thesmaller the molecular weight.

The above SSG can be adjusted by polymerization conditions(polymerization pressure, etc.) at the time of producing the modifiedPTFE.

SSG of the modified PTFE is measured in accordance with ASTM D4895-04.

The state of the modified PTFE is preferably particulate from theviewpoint of e.g. handling efficiency.

The average primary particle size of the modified PTFE particles ispreferably from 0.10 to 0.50 μm, more preferably from 0.15 to 0.30 μm,further preferably from 0.20 to 0.30 μm. When the average primaryparticle size is at least 0.10 μm, paste extrusion molding can becarried out at a low extrusion pressure, and a molded product excellentin surface smoothness having no such undulation on the surface, can beeasily obtained. When the average primary particle size is at most 0.50μm, spaces among the particles during extrusion tend to be reduced,whereby extrusion stability will be excellent, and as a result, a moldedproduct excellent in surface smoothness can be easily obtained.

The average primary particle size of the modified PTFE particlescorresponds to D50 measured by, for example, a laser scattering methodparticle size distribution analyzer. As will be described later, in thecase of producing the modified PTFE in an aqueous medium, by using theobtainable aqueous dispersion of the modified PTFE particles, the abovemeasurement may be carried out to obtain the average primary particlesize of the modified PTFE particles.

The extrusion pressure for the modified PTFE is preferably from 18.0 to35.0 MPa, more preferably from 20.0 to 25.0 MPa, from such a viewpointthat the paste extrusion will thereby be easy.

Measurement of the Extrusion Pressure is as Follows.

A sample (modified PTFE) (100 g) left to stand at room temperature forat least 2 hours, is put into a glass bottle having an internal capacityof 500 mL, and 21.7 g of a lubricating oil (Isopar H (registeredtrademark), manufactured by Exxon Corporation) is added and mixed for 3minutes to obtain a mixture. The obtained mixture is left to stand in a25° C. thermostatic bath for 2 hours, and then subjected to pasteextrusion through an orifice at an introduction angle 30° with adiameter of 2.5 cm and a land length of 1.1 cm at 25° C. underconditions of a reduction ratio (ratio of the cross-sectional area ofthe inlet to the cross-sectional area of the outlet of the die) of 100and an extrusion speed of 51 cm/min, to obtain an extruded bead(cord-like material). The pressure required for extrusion at that timeis measured and adopted as the extrusion pressure (unit: MPa).

The breaking strength A of the modified PTFE is preferably at least5.0N, more preferably at least 8.0N. The breaking strength A of themodified PTFE is usually at most 50N.

Measurement of the Breaking Strength A is as Follows.

An extruded bead is obtained in the same manner as in the method formeasurement of the extrusion pressure, and it is dried at 230° C. for 30minutes, to remove the lubricant. Then, the extruded bead is cut into asuitable length, and both ends are fixed so that the clamp intervalbecomes to be 5.1 cm, followed by heating to 300° C. in an aircirculating oven. Continuously, stretching is carried out underconditions of a stretching rate of 100%/sec and a stretching ratio of2,400%, to obtain a modified PTFE stretched porous material (hereinafterreferred to as a stretched bead).

With respect to samples from the respective ends of the stretched bead(if there is any neck-down in the clamped ranges, excluding suchneck-down) and a sample obtainable from the center of the stretchedbeading, i.e. a total of three samples, the tensile breaking load forcesare, respectively, measured by using a tensile tester (manufactured byA&D Company, Limited), whereby the minimum value is adopted as thebreaking strength A.

In the measurement by the tensile tester, the sample is sandwiched andfixed by movable jaws with a gauge length of 5.0 cm, and the movablejaws are driven at room temperature (24° C.) at a speed of 300 mm/min,to impart a tensile stress.

The stress relaxation time of the modified PTFE is preferably at least100 seconds, more preferably at least 110 seconds, further preferably atleast 115 seconds, from such a viewpoint that the heat resistance of themodified PTFE will be more excellent. The stress relaxation time of themodified PTFE is usually at most 700 seconds.

Measurement of the Stress Relaxation Time is as Follows.

Under conditions of a clamping interval of 3.8 cm, a stretching rate of1,000%/sec and a total stretching of 2,400%, in the same manner as inthe measurement of the breaking strength, an extruded bead is stretched,and both ends of a sample of this stretched bead obtained are fixed byjigs, whereby the time required for the sample to be broken when left tostand in an oven of 390° C. is obtained.

The breaking strength B of the modified PTFE to be measured by themethod as described later, is preferably at least 21.0N, more preferablyat least 23.0N. The breaking strength B of the modified PTFE is usuallyat most 70N.

Measurement of the Breaking Strength B is as Follows.

In the same manner as in the evaluation of the stress relaxation time,an extruded bead is stretched to obtain the same sample as the samplefor measurement of the stress relaxation time. Then, using the obtainedsample, the same measurement as the measurement of the breaking strengthis conduced to obtain the value of the breaking strength B.

As the method for producing the above modified PTFE, a known method maybe adopted. However, in a known method, particularly in a knownproduction method using an emulsion polymerization method, a fluorinatedsurfactant is usually used. But, as described above, from the viewpointof environmental problems, it is preferred to produce the modified PTFEwithout using a fluorinated surfactant.

The modified PTFE of the present invention is preferably a modified PTFEproduced without using a fluorinated surfactant, i.e. a modified PTFEobtained by polymerizing TFE in the absence of a fluorinated surfactant.

Further, the modified PTFE of the present invention containing nofluorinated surfactant, is preferably a modified PTFE obtained bypolymerizing TFE in an aqueous medium in which a polymer A containingunits A is present.

The above aqueous medium in which a polymer A is present, is preferablyan aqueous medium in which a polymer A is present which is obtained bypolymerizing a monomer A in the aqueous medium. In each of thepolymerization of a monomer A in an aqueous medium and thepolymerization of TFE in an aqueous medium in which a polymer A ispresent, by conducting the polymerization in the absence of afluorinated surfactant, it is possible to produce a modified PTFEcontaining no fluorinated surfactant.

As one of preferred embodiments of the method for producing a modifiedPTFE, an embodiment having the following two steps may be mentioned.

Step 1: a step of conducting polymerization of a monomer A in an aqueousmedium, to obtain the aqueous medium containing a polymer A

Step 2: a step of conducting polymerization of TFE in the aqueous mediumin which the polymer A is present, to obtain a modified PTFE.

In the following, the procedures of the respective steps will bedescribed in detail.

<Step 1>

Step 1 is a step of conducting polymerization of a monomer A in anaqueous medium, to obtain the aqueous medium containing a polymer A.

In the following, first, materials to be used in step 1 will bedescribed in detail, and then, the procedure in step 1 will be describedin detail.

The definition of a monomer A is as described above.

(Aqueous Medium)

The aqueous medium may, for example, be water, or a mixture of water anda water-soluble organic solvent.

The water-soluble organic solvent may, for example, be tert-butanol,propylene glycol or dipropylene glycol. The aqueous medium is preferablycomposed of water only.

(Polymerization Initiator)

In step 1, a polymerization initiator may be used. That is, at the timeof the polymerization of a monomer A, a polymerization initiator may beused.

As the polymerization initiator, a water-soluble radical initiator or awater-soluble redox catalyst is preferred.

The water-soluble radical initiator is preferably a persulfate such asammonium persulfate or potassium persulfate, or a water-soluble organicperoxide such as disuccinic acid peroxide, bisglutaric acid peroxide,tert-butyl hydroperoxide or the like.

The water-soluble redox catalyst is preferably a combination of anoxidizing agent, such as a bromic acid or a salt thereof, a chloric acidor a salt thereof, a persulfuric acid or a salt thereof, a permanganicacid or a salt thereof, or hydrogen peroxide, and a reducing agent, suchas sulfurous acid or a salt thereof, hydrogen sulfide or a salt thereof,thiosulfate or a salt thereof, or an organic acid. Among them, acombination of a bromic acid or a salt thereof, and sulfurous acid or asalt thereof, or ammonium sulfite, or a combination of permanganic acidor a salt thereof, or potassium permanganate, and oxalic acid, is morepreferred.

As the polymerization initiator, ammonium persulfate alone or a mixedsystem of a persulfate and disuccinic acid peroxide, is preferred;ammonium persulfate alone or a mixed system of ammonium persulfate anddisuccinic acid peroxide, is more preferred; and ammonium persulfatealone is further preferred.

As the polymerization initiator, one type may be used alone, or two ormore types may be used in combination.

Further, as the method for charging the polymerization initiator, theentire amount may be charged to the polymerization system beforeinitiating the polymerization reaction, or it may be added continuouslyor intermittently to the polymerization system.

(Procedure in Step)

In step 1, the polymerization of a monomer A is conducted in an aqueousmedium. Specifically, it is preferred to mix the monomer A and theaqueous medium, and to conduct the polymerization of the monomer A inthe obtained mixture.

The amount (charging amount) of the monomer A is adjusted so that thecontent of units A becomes to be from 10 to 500 mass ppm, to all unitsin the obtainable modified PTFE.

Here, the method for charging the monomer A is preferably initial batchaddition wherein the entire amount is charged to the polymerizationsystem in advance before initiating the polymerization reaction.

The content of the monomer 1 in the solution obtainable by mixing themonomer 1 and the aqueous medium is preferably from 0.0005 to 0.0080mass %, more preferably from 0.0005 to 0.0030 mass %, to the total massof the solution.

The amount of the polymerization initiator to be used is preferably from0.2 to 1,000 mass %, more preferably from 0.2 to 500 mass %, to theentire amount of the monomer A.

The polymerization temperature of the monomer A is preferably from 10 to95° C., more preferably from 50 to 90° C. The polymerization time ispreferably from 5 to 400 minutes, more preferably from 5 to 300 minutes.

The pressure condition at the time of the polymerization is preferably avacuum condition or an atmospheric condition.

Further, by adjusting the atmosphere at the time of the polymerizationto be a TFE atmosphere, the polymerization may be carried out. Here,usually, polymerization of the monomer A in an aqueous medium proceedsin preference to polymerization of TFE.

In the above step 1, an aqueous medium containing a polymer A isobtained, in which particles of the polymer A are dispersed in theaqueous medium. At the time of polymerization of TFE in the laterdescribed step 2, although the particles of the polymer A are not anemulsifier, due to the balance of interfacial tensions to both of theaqueous medium and the modified PTFE particles during polymerization,particles of the polymer A are considered to be present at the boundaryof the both and thus to contribute to the dispersion stabilization inthe aqueous medium of the modified PTFE particles. Particles of themodified PTFE obtained by step 2 are particles containing particles ofthe polymer A.

The particle size of particles of the polymer A is preferably from 0.1to 100 nm, more preferably from 0.1 to 50 nm.

Particles of the polymer A are composed of a polymer comprising units A.

The polymer A usually comprises only units A, but may contain unitsbased on a fluorinated monomer within a range not to impair the effectsof the present invention. The fluorinated monomer is a monomer havingfluorine atoms, and, for example, TFE may be mentioned.

The content of units A in the polymer A is preferably at least 90 mass%, more preferably at least 95 mass %, to all units in the polymer A. Asthe upper limit, 100 mass % may be mentioned.

<Step 2>

Step 2 is a step of conducting polymerization of TFE in the aqueousmedium in which the polymer A is present, to obtain a modified PTFE.

In the following, first, the materials to be used in step 2 will bedescribed in detail, and then, the procedure in step 2 will be describedin detail.

(Polymerization Initiator)

In step 2, a polymerization initiator may be used. That is, apolymerization initiator may be used at the time of the polymerizationof TFE.

The polymerization initiator to be used may be a polymerizationinitiator as described in step 1.

As the polymerization initiator, a mixed system of a persulfate anddisuccinic acid peroxide, is preferred, and a mixed system of ammoniumpersulfate and disuccinic acid peroxide is more preferred.

The amount of the polymerization initiator to be used is preferably atleast 0.10 mass %, more preferably from 0.10 to 1.5 mass %, furtherpreferably from 0.20 to 1.0 mass %, to the total amount of TFE to besupplied to the polymerization system.

(Surfactant)

In step 2, together with the polymer A, it is preferred to use afluorine-free surfactant. That is, it is preferred to carry out thepolymerization of TFE in the presence of a fluorine-free surfactanttogether with the polymer A.

The fluorine-free surfactant is a surfactant having a hydrophobicportion composed of an organic group containing no fluorine atoms. Thefluorine-free surfactant preferably contains no fluorine atoms in aportion other than the hydrophobic portion such as a hydrophilicportion, etc.

As the fluorine-free surfactant, a hydrocarbon type surfactant ispreferred. The hydrocarbon type surfactant is a surfactant wherein thehydrophobic portion is made of a hydrocarbon. The hydrocarbon typesurfactant may be any of anionic, nonionic and cationic, and ahydrocarbon type anionic surfactant is preferred. Here, in the abovehydrocarbon, an oxygen atom (—O—) may be contained. That is, it may be ahydrocarbon containing oxyalkylene units.

The number of carbon atoms contained in the above hydrocarbon group ispreferably from 5 to 20.

The counter cation to the anion of the hydrocarbon type anionicsurfactant may, for example, be H⁺, Na⁺, K⁺, NH₄ ⁺, NH(EtOH)₃ ⁺ or thelike.

The hydrocarbon type anionic surfactant may, for example, be sodiumdodecyl sulfate, sodium lauryl sulfate, ammonium lauryl sulfate,polyoxyethylene lauryl ether sodium sulfate, polyoxyethylene laurylether ammonium sulfate, sodium dodecylbenzenesulfonate, sodiumdi(2-ethylhexyl)sulfosuccinate, sodium laurate, or ammonium laurate.

As the hydrocarbon type surfactant, one type may be used alone, or twoor more types may be used in combination.

(Aqueous Medium)

As the aqueous medium containing the polymer A in step 2, an aqueousmedium containing the polymer A obtained in step 1, or an aqueous mediumcontaining the polymer A, which is obtainable by diluting the aqueousmedium containing the polymer A obtained in step 1 with an aqueousmedium, may be used. The aqueous medium for dilution may be the sameaqueous medium as the aqueous medium used in step 1, or may be adifferent aqueous medium.

(Stabilizing Aid)

In step 2, a stabilizing aid which is commonly used in emulsionpolymerization for PTFE may be used. The stabilizing aid is not onewhich brings about interference with polymerization of the monomer A instep 1, and therefore, by letting it be present in the aqueous medium tobe used in step 1, polymerization of the monomer A may be conducted, andthe obtained polymer A-containing aqueous medium containing thestabilizing aid, may be used in step 2.

As the stabilizing aid, paraffin wax, a fluorinated solvent or siliconeoil is preferred, and paraffin wax is more preferred. The paraffin waxmay be liquid, semi-liquid or solid at room temperature. Especially, asaturated hydrocarbon having at least 12 carbon atoms, is preferred. Themelting point of the paraffin wax is preferably from 40 to 65° C., morepreferably from 50 to 65° C.

As the stabilizing aid, one type may be used alone, or two or more typesmay be used in combination.

(Other)

Further, in step 2, monomers other than TFE may be used within a rangenot to impair the effects of the present invention, but from such aviewpoint that various characteristics of modified PTFE will be moreexcellent, the total amount of TFE is preferably at least 99.5 mass % tothe total amount of monomers to be used in step 2. Especially, it ismore preferred to use only TFE as the monomer in step 2.

(Procedure in Step)

In a usual way, TFE is introduced into the polymerization system (thatis, the polymerization reaction vessel). Specifically, TFE is introducedcontinuously or intermittently into the polymerization system so thatthe polymerization pressure becomes to be a predetermined pressure.

In a case of using a polymerization initiator, the polymerizationinitiator may be added all at once to the polymerization system, or maybe added dividedly.

The polymerization temperature of TFE is preferably from 10 to 95° C.,more preferably from 15 to 90° C. The polymerization pressure ispreferably from 0.5 to 4.0 MPa, more preferably from 0.6 to 3.5 MPa. Thepolymerization time is preferably from 90 to 520 minutes, morepreferably from 90 to 450 minutes.

Further, step 1 and step 2 may be carried out continuously in the samepolymerization reaction vessel.

Further, in the production method of the present invention, it issufficient that particles of the polymer A are formed in step 1, andstep 2 may be conducted before the monomer A is completely consumed instep 1.

By the above procedure, an aqueous dispersion having modified PTFEdispersed in the form of particles (aqueous dispersion containingmodified PTFE particles) is obtainable. The concentration of themodified PTFE particles in the aqueous dispersion is preferably from 10to 45 mass %, more preferably from 15 to 45 mass %, further preferablyfrom 20 to 43 mass %. Within the above range, the modified PTFEparticles in the aqueous dispersion can be more easily coagulated, andwhite turbidity of the coagulation liquid can be suppressed.

The preferable range of the average primary particle size of themodified PTFE particles is as described above.

In the foregoing, an embodiment of conducting step 1 has been described,but another method may be used, so long as polymerization of TFE isconducted in an aqueous medium in the presence of particles of thepolymer A. For example, it may be a method wherein particles of thepolymer A separately prepared, are added into an aqueous medium, andthen, polymerization of TFE is conducted in the aqueous medium.

<Modified PTFE Powder>

By the above-described procedure of the production method, an aqueousdispersion containing modified PTFE is obtainable.

Here, a method of obtaining a modified PTFE powder composed of modifiedPTFE particles (modified PTFE fine powder) from the aqueous dispersioncontaining modified PTFE particles may, for example, be a method ofcoagulating modified PTFE particles.

Specifically, for example, by diluting with water so that theconcentration of the modified PTFE in the aqueous dispersion containingmodified PTFE particles becomes to be from 8 to 25 mass %, thetemperature of the aqueous dispersion is adjusted to from 5 to 35° C.,and then, the aqueous dispersion is vigorously stirred to coagulatemodified PTFE particles. At that time, the pH may be adjusted as thecase requires. Further, a coagulating aid such as an electrolyte or awater-soluble organic solvent may be added to the aqueous dispersion.

Thereafter, moderate agitation is conducted, and the coagulated modifiedPTFE particles are separated from water, and the obtained wet powder(wet fine powder) is granulated and sieved, as the case requires,followed by drying as the case requires. Thus, a modified PTFE powder isobtainable.

The above drying is conducted in a state of not letting the wet powderflow so much, preferably by letting it be left to stand still. Thedrying method may, for example, be vacuum drying, radio frequency dryingor hot air drying.

The drying temperature is preferably from 10 to 300° C., more preferablyfrom 100 to 250° C.

Especially, the drying of the undried modified PTFE powder is preferablycarried out in an atmosphere containing ammonia. Here, the atmospherecontaining ammonia means an atmosphere in which ammonia gas can be incontact with the undried modified PTFE powder. For example, it means anatmosphere containing ammonia gas, or an atmosphere in which ammonia ora compound which generates ammonia, is dissolved in water containing theundried modified PTFE powder, and ammonia gas will be generated byheating.

The compound which generates ammonia may, for example, be an ammoniumsalt or urea. Such a compound will be decomposed by heating to generateammonia gas.

When the undried modified PTFE powder is dried in an atmospherecontaining ammonia, without impairing the physical properties, it ispossible to lower the paste extrusion pressure of the modified PTFEpowder.

<Molded Product>

The modified PTFE as described above is suitably applicable for pasteextrusion molding.

The modified PTFE (especially, the modified PTFE powder) is subjected topaste extrusion molding, whereby a desired molded product will beobtained.

The paste extrusion molding is a method in which the modified PTFEpowder and a lubricant are mixed, to let the modified PTFE powder have afluidity, and the mixture is extrusion molded, to form a molded productof, for example, a film or a tube.

The mixing proportion of the lubricant may be suitably selected so thatthe modified PTFE powder has a fluidity, and, for example, when thetotal amount of the modified PTFE powder and the lubricant is 100 mass%, it is preferably from 10 to 30 mass %, more preferably from 15 to 20mass %.

As the lubricant, for example, naphtha or a petroleum type hydrocarbonhaving a dry point of at least 100° C. is preferred.

To the mixture, an additive such as a pigment may be added for thepurpose of coloration, or various fillers may be added for the purposeof imparting e.g. strength and conductivity.

The shape of the molded product may, for example, be tubular, sheet,film or fibrous. Applications may, for example, be tubes, covering forelectric wires, sealing materials, porous membranes and filters.

Further, the modified PTFE powder may be paste-extruded to obtain anextruded bead, and the extruded bead may be stretched to obtain astretched porous material of modified PTFE. The stretching conditionsmay, for example, be a rate of from 5 to 1,000%/sec and a stretchingratio of at least 500%.

The shape of an article made of the stretched porous material may, forexample, be tubular, sheet, film or fibrous.

EXAMPLES

In the following, the present invention will be described in more detailwith reference to Examples and Comparative Examples, but the presentinvention is not limited thereto.

Various measuring methods and evaluation methods are as follows.

(A) Average Primary Particle Size (Nm) of Modified PTFE Particles(Hereinafter Referred to Also as “PPS”)

Measured by means of a laser scattering method particle sizedistribution analyzer (manufactured by Horiba, Ltd., trade name“LA-920”) by using an aqueous dispersion of modified PTFE particles as asample.

(B) Standard Specific Gravity (SSG)

Measured in accordance with ASTM D4895-04.

12.0 g of a sample (modified PTFE powder) was weighed, and held in acylindrical mold having an inner diameter of 28.6 mm under 34.5 MPa for2 minutes. This sample was put in an oven of 290° C. and heated at 120°C./hr. Further, after maintaining at 380° C. for 30 minutes, thetemperature was lowered at 60° C./hr and held at 294° C. for 24 minutes.The sample was held in a desiccator of 23° C. for 12 hours, then, thespecific gravity value to water of the sample at 23° C. was measured,and this value was adopted as the standard specific gravity. As thevalue of SSG is small, the molecular weight is large.

(C) Measurement of Extrusion Pressure

A modified PTFE powder (100 g) left to stand at room temperature for atleast 2 hours, was put into a glass bottle having an internal capacityof 500 mL, and a lubricating oil (Isopar H (registered trademark),manufactured by Exxon Corporation) (21.7 g) was added and mixed for 3minutes to obtain a mixture. The obtained mixture was left to stand in a25° C. thermostatic bath for 2 hours, and then subjected to pasteextrusion through an orifice at an introduction angle of 30° with adiameter of 2.5 cm and a land length of 1.1 cm at 25° C. underconditions of a reduction ratio (ratio of the cross-sectional area ofthe inlet to the cross-sectional area of the outlet of the die) of 100and an extrusion speed of 51 cm/min, to obtain an extruded bead(cord-like material). The pressure required for extrusion at that timewas measured and adopted as the extrusion pressure (unit: MPa).

(D) Measurement of Breaking Strength A

An extruded bead was obtained in the same manner as in the measurementof the extrusion pressure, and it was dried at 230° C. for 30 minutes,to remove the lubricant. Then, the extruded bead was cut into a suitablelength, and both ends were fixed so that the clamp interval became 5.1cm, followed by heating to 300° C. in an air circulating oven.Continuously, stretching was conducted under conditions of a stretchingrate of 100%/sec and a stretching ratio of 2,400%, to obtain a modifiedPTFE stretched porous material (hereinafter referred to as a stretchedbead).

With respect to samples obtainable from the respective ends of thestretched bead (if there is any neck-down in the clamped region,excluding the neck-down), and a sample obtainable from the center of thestretched bead, i.e. a total of three samples, the tensile breaking loadforces were, respectively, measured, by means of a tensile tester(manufactured by A&D Company, Limited), and the minimum value wasadopted as the breaking strength A.

In the measurement by the tensile tester, the sample was sandwiched andfixed by movable jaws having a gauge length of 5.0 cm, and the movablejaws were driven at room temperature (24° C.) at a speed of 300 mm/minto impart a tensile stress.

(E) Measurement of Stress Relaxation Time

Under conditions of a clamp interval of 3.8 cm, a stretching rate of1,000%/sec and a total stretching of 2,400%, in the same manner as inthe measurement of the breaking strength, an extruded bead wasstretched, to prepare a sample for measurement of the stress relaxationtime. Both ends of this sample were fixed by fixtures and pulled taut tomake the total length to be 25 cm. The stress relaxation time wasobtained by the time required for this sample to be broken when it wasleft in an oven of 390° C.

(F) Measurement of Breaking Strength B

In the same manner as in the evaluation of the stress relaxation time,the extruded bead was stretched to produce the same sample as the samplefor measurement of the stress relaxation time. Then, the samemeasurement as the measurement of the breaking strength was conducted toobtain the value of breaking strength B.

Example 1

In a 100 L stainless steel autoclave, paraffin wax (1,500 g) anddeionized water (60 L) were charged. The autoclave was purged withnitrogen and then brought to a reduced pressure, and n-butylmethacrylate (1 g) and deionized water (0.5 L), were poured and chargedinto the autoclave. Here, n-butyl methacrylate was charged, so that thecontent of units based on n-butyl methacrylate became to be 48 mass ppmto all units of the obtainable modified PTFE.

Next, inside of the autoclave was brought to a state of at most theatmospheric pressure, and while stirring the solution in the autoclave,the temperature was raised to 75° C. Thereafter, a solution prepared bydissolving ammonium persulfate (0.11 g) as a polymerization initiator indeionized water (1 L), was injected into the autoclave, to polymerizen-butyl methacrylate.

10 minutes later, the pressure was raised to 1.96 MPa by TFE, and asolution prepared by dissolving ammonium persulfate (0.54 g) anddisuccinic acid peroxide (concentration 80 mass %, remainder water) (53g) in warm water (1 L) of about 70° C., was injected into the autoclave.By 1,379 seconds later, the internal pressure in the autoclave waslowered to 1.89 MPa. Here, the amount of the polymerization initiators(ammonium persulfate and disuccinic acid peroxide) used, was 0.26 mass %to the total amount of TFE used.

Then, TFE was added so as to keep the inner pressure of the autoclave tobe 1.96 MPa, to let polymerization of TFE proceed. After adding 1 kg ofTFE, a solution prepared by dissolving dodecyl sodium sulfate (44 g) indeionized water (3 L), was supplied while confirming the amount of TFEto be supplied by a flow meter, so that sodium dodecyl sulfate becomesto be from 1.5 to 1.6 g to 1 kg of TFE supplied.

At the time when the amount of added TFE reached 21 kg, the reaction wasterminated, and TFE in the autoclave was released to the atmosphere. Thepolymerization time was 226 minutes.

The obtained aqueous dispersion of modified PTFE was cooled, and thesupernatant paraffin wax was removed. The solid content concentration ofthe aqueous dispersion (concentration of modified PTFE) was about 23mass %. Further, the average primary particle size of modified PTFE inthe aqueous dispersion was 260 nm.

The aqueous dispersion was diluted with pure water to a solid contentconcentration of 10 mass %, adjusted to 20° C. and stirred, to letmodified PTFE particles be coagulated, to obtain a modified PTFE powder.Then, this modified PTFE powder was dried at 250° C.

SSG of the obtained modified PTFE powder was 2.162. The extrusionpressure was 21.6 MPa. The breaking strength was 21.1N. The stressrelaxation time was 180 seconds.

Example 2

An aqueous dispersion of modified PTFE was obtained in accordance withthe same procedure as described above (Example 1), except that insteadof the treatment in which inside of the autoclave was adjusted to astate of at most the atmospheric pressure, and the solution in theautoclave was heated with stirring to 75° C., treatment was conducted inwhich inside of the autoclave was pressurized to 0.15 MPa by TFE, andthe solution in the autoclave was heated with stirring to 75° C.

Various evaluations are summarized in Table 1.

Example 3

A modified PTFE powder was obtained in accordance with the sameprocedure as in Example 1, except that n-butyl methacrylate was chargedso that the content of units based on n-butyl methacrylate became to be56 mass ppm, to all units in the obtainable modified PTFE, and theamount of TFE used, was changed from 21 kg to 18 kg.

Various evaluations are summarized in Table 1.

Example 4

A modified PTFE powder was obtained in accordance with the sameprocedure as in Example 3, except that n-butyl methacrylate was changedto vinyl acetate.

Various evaluations are summarized in Table 1.

Example 5

A modified PTFE powder was obtained in accordance with the sameprocedure as in Example 1, except that n-butyl methacrylate was chargedso that the content of units based on n-butyl methacrylate became to be83 mass ppm to all units in the obtainable modified PTFE, and the amountof TFE used, was changed from 21 kg to 12 kg.

Various evaluations are summarized in Table 1.

Example 6

A modified PTFE powder was obtained in accordance with the sameprocedure as in Example 5, except that n-butyl methacrylate was changedto vinyl acetate.

Various evaluations are summarized in Table 1.

Example 7

A modified PTFE powder was obtained in accordance with the sameprocedure as in Example 1, except that n-butyl methacrylate was changedto acrylic acid; acrylic acid was charged so that the content of unitsbased on acrylic acid became to be 100 mass ppm to all units in theobtainable modified PTFE; and the amount of TFE used, was changed from21 kg to 10 kg.

Various evaluations are summarized in Table 1.

Example 8

At the time of diluting the aqueous emulsion of modified PTFE in Example2 with pure water to a concentration of 10 mass %, and then adjusting itto 20° C., followed by stirring to coagulate it, the coagulation wasconducted by charging 5 mass % of ammonium carbonate to modified PTFE inthe coagulation vessel. Then, the water content of the obtained undriedmodified PTFE fine powder was measured, and based on that value, in adrying tray, the undried modified PTFE fine powder, and an ammoniumcarbonate aqueous solution (ammonium carbonate concentration of 20 mass%) to be 5 mass % to the modified PTFE, were simultaneously put, and theobtained drying tray was dried at 285° C.

Using the obtained sample, measurements of the breaking strength and thestress relaxation time were conducted. The breaking strength A was28.2N, and the breaking strength B was 33.8N. The stress relaxation timewas 176 seconds.

Comparative Example 1

A modified PTFE powder was obtained in accordance with the sameprocedure as in Example 5, except that n-butyl methacrylate was changedto vinyl sulfonic acid.

Various evaluations are summarized in Table 1.

In Table 1, “BMA” represents n-butyl methacrylate, “VAc” representsvinyl acetate, “AA” represents acrylic acid, and “VSa” representsvinylsulfonic acid.

The “content (mass ppm)” represents the content of units A to all unitsin the obtainable modified PTFE.

The “solids (mass %)” represents the solid content concentration of theaqueous dispersion (concentration of modified PTFE).

TABLE 1 Example Example Example Example Example Example ExampleComparative 1 2 3 4 5 6 7 Example 1 Monomer A Type BMA BMA BMA VAc BMAVAc AA VSa Content (mass ppm) 48 48 56 56 83 83 100 83 TFEPolymerization time (min) 226 214 171 170 116 115 148 149 polymerizationTFE supply amount (kg) 21 21 18 18 12 12 10 12 conditions Various Solids(mass %) 23.3 23.5 20.7 21.4 15.4 15.4 12.9 15.7 evaluations Averageprimary particle size 0.26 0.26 0.24 0.29 0.20 0.24 0.24 0.17 (μm)Standard specific gravity 2.162 2.163 2.164 2.163 2.169 2.167 2.1672.176 Extrusion pressure (MPa) 21.6 22.9 23.3 21.9 24.0 23.0 23.9 25.7Breaking strength A (N) 21.1 20.8 18.8 17.3 8.7 19.9 16.8 Notstretchable Stress relaxation time (sec) 180 174 157 150 119 120 144 99Breaking strength B (N) 26.5 27.5 28.4 23.3 27.8 25.1 25.5 20.4

As shown in Table 1, the modified PTFE of the present invention wasexcellent in breaking strength.

This application is a continuation of PCT Application No.PCT/JP2018/035492, filed on Sep. 25, 2018, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2017-187990 filed on Sep. 28, 2017. The contents of those applicationsare incorporated herein by reference in their entireties.

What is claimed is:
 1. A modified polytetrafluoroethylene comprising apolymer having units based on tetrafluoroethylene, and a polymer havingunits based on a monomer represented by the formula (1), wherein thecontent of the units based on a monomer represented by the formula (1)is from 10 to 500 mass ppm, to all units in the modifiedpolytetrafluoroethylene, and the standard specific gravity is from 2.155to 2.175,CH₂═CR¹-L-R²  Formula (1) wherein R¹ represents a hydrogen atom or analkyl group, L represents a single bond, —CO—O—*, —O—CO—* or —O—, *represents a bonding position to R², and R² represents a hydrogen atom,an alkyl group or a nitrile group.
 2. The modifiedpolytetrafluoroethylene according to claim 1, wherein the content of theunits based on a monomer represented by the formula (1) is from 30 to100 mass ppm, to all units in the polytetrafluoroethylene.
 3. Themodified polytetrafluoroethylene according to claim 1, wherein themonomer represented by the formula (1) is a monomer selected from thegroup consisting of a monomer represented by the formula (1-1), amonomer represented by the formula (1-2), a monomer represented theformula (1-3), and a monomer represented by the formula (1-4),CH₂═CR¹—CO—O—R³  Formula (1-1)CH₂═CR¹—O—CO—R⁴  Formula (1-2)CH₂═CR¹—O—R⁵  Formula (1-3)CH₂═CR¹—R⁶  Formula (1-4) wherein R¹ represents a hydrogen atom or analkyl group, R³ represents a hydrogen atom or an alkyl group, R⁴represents an alkyl group, R⁵ represents an alkyl group, and R⁶represents a nitrile group.
 4. The modified polytetrafluoroethyleneaccording to claim 1, wherein the monomer represented by the formula (1)is a monomer represented by the formula (1-1).
 5. The modifiedpolytetrafluoroethylene according to claim 1, wherein the modifiedpolytetrafluoroethylene is in a particulate form, and its averageprimary particle size is from 0.20 to 0.30 μm.
 6. The modifiedpolytetrafluoroethylene according to claim 1, which is for pasteextrusion molding.
 7. A molded product formed by paste extrusion moldingof the modified polytetrafluoroethylene as defined in claim
 1. 8. Amethod for producing a stretched porous material, which comprisessubjecting the modified polytetrafluoroethylene as defined in claim 1 topaste extrusion to obtain an extruded bead, and stretching the extrudedbead to obtain a stretched porous material.